1. Field of the Invention
The present invention relates to a method of determining at least one geometrical-and-postural parameter for placing a frame of vision-correcting eyeglasses on the face of a wearer in the anatomical posture.
2. Description of the Related Art
In the context of mounting vision-correcting ophthalmic lenses in an eyeglass frame, it is presently preferred to take best account of individual geometrical-and-postural parameters associated with the wearer and the frame selected by the wearer, which parameters are referred to as personalized optical design parameters.
The looked-for geometrical-and-postural parameters are associated both with the shapes of the wearer's head and of the selected eyeglass frame, and also with the wearer's posture. They comprise in particular the three-dimensional configuration of each lens relative to the wearer's head under wearing conditions. This three-dimensional configuration is determined in particular by the pantoscopic angle of each lens when worn (the angle formed between the general plane of the lens relative to the vertical) and the heights of the wearer's eyes relative to the bottom edges of the lenses.
In order to determine this three-dimensional configuration, the optician places a pair of presentation eyeglasses on the wearer's nose. The presentation eyeglasses comprise the frame selected by the wearer together with non-correcting lenses mounted in the rims of the frame.
The height of the wearer's eyes relative to the bottom edge of the frame can be measured manually: the optician faces the wearer from in front and uses a rule to estimate a measurement of the distance between the pupil of an eye and the bottom edge of the presentation lens.
Proposals have been made to automate determining these geometrical-and-postural parameters on the basis of one or more captured images of the wearer wearing the selected eyeglass frame. The images are processed in order to determine the looked-for parameters.
Nevertheless, those methods of determining the geometrical-and-postural parameters of the wearer, regardless of whether they are manual or automated, suffer from considerable inaccuracy. In order to be able to obtain an accurate measurement whether by using a rule or by processing captured images, it is necessary for the wearer's head to be in the anatomical posture when the measurement is made or the images are captured. This anatomical posture is also referred to as the orthostatic position.
Unfortunately, for reasons of speed of execution and of comfort for the optician and for the wearer, the position of the wearer's head while taking the image is not monitored. However, if the angle of inclination of the head in its sagittal plane departs from the anatomical posture by 1 degree, there will be an error of 1 degree in the measurement of the pantoscopic angle and an error of 0.5 millimeters in the measurement of the heights of the eyes.